Furfuraldehyde detector and method of manufacturing the same

ABSTRACT

Sensing the presence of furfuraldehyde (FFA) in oil is effected by a porous solid sensor ( 10 ) comprising aniline acetate entrapped in an inert matrix. The matrix may be a sol-gel preferably based on methyltrimethoxysilane. The solid sensor ( 10 ) reacts with FFA to form a pink colored complex which has a characteristic which correlates with the concentration of FFA in the oil and which can be measured photo-optically. Photo-optical measurement may be by absorption or fluorescence when the sensor is irradiated with an interrogating beam. The absorption maximum wavelength is about 525 nm and irradiation at that wavelength produces a maximum fluorescent output at about 570 nm.

The present invention relates to detection of furfuraldehyde (FFA) intransformer oil.

The lifetime of a transformer is often limited by the ageing ordegradation (polymerisation) of the paper insulation which is used onthe transformer windings. Indirect methods of examining the paperinsulation inside a transformer have to be used because it isimpractical to examine the paper insulation inside a transformerdirectly.

A known method of determining the quality of the paper insulation relieson the fact that the thermal ageing process of the paper is accompaniedby the evolution of several paper by-products into the transformer oilin which the transformer core is immersed. These by-products includegases such as CO and CO₂ and a chemical compound called furfuraldehyde.

The gases CO and CO₂ can be produced by the thermal degradation of theoil itself. Furfuraldehyde, however, is only produced by degradation ofthe paper, so its detection can provide an early indication of theoverheating of the paper insulation.

The current practice involves periodically collecting samples of oilfrom the transformer or transformers to be tested, transporting thesamples to a laboratory, usually remote from the transformers, andanalysing the samples using a colourimetric process.

The colourimetric process relies on the specific reaction between FFAand the compound aniline acetate which yields a complex with a brightpink colour the intensity of which is a characteristic which can bemeasured photo-optically and which correlates with the concentration ofFFA in the transformer oil. The liquid reagents used during the chemicalanalysis (and the fumes produced) are toxic if ingested and thereforegreat care must be taken during the laboratory chemical analysis.

Present practices therefore do not enable an on-the-spot assessment (orestimate) of the state of the paper insulation via FFA measurements tobe made. The analyses are conducted in a chemical laboratory, sometimesdays after the oil samples were actually extracted from thetransformers. Thus the analysis is slow and expensive. This procedurealso introduces a long time delay (for example one week) between takinga sample of oil from a transformer containing faulty paper insulationand determining that the paper insulation is faulty. The effect of thisdelay is that the transformer is in use for the delay period (one week)even though the paper insulation is faulty and in need of replacement.

It is an object of the present invention to mitigate or obviate one ormore of the above disadvantages.

This is achieved by entrapping aniline acetate which is indicative ofthe state of the paper in an inert matrix to produce a porous solidsensor which facilitates on-site detection of paper degradation withoutexposing the user to toxic chemicals.

According to a first aspect of the present invention there is provided asolid sensor for detecting furfuraldehyde in oil, where the sensorcomprises a solidified matrix in which aniline acetate is entrapped, andwhere the matrix is made of an inert material which allows ingress offurfuraldehyde from the oil to react with the aniline acetate to yieldan entrapped complex having a characteristic which is photo-opticallymeasurable and which correlates with the concentration of furfuraldehydein the oil.

Preferably, the matrix material is a sol-gel.

The sold sensor may have sufficient thickness to be self-supporting orit may take the form of a thin film or coating carried by a substratewhich may be transparent (e.g. glass), or optically reflective to permitphoto-optic measurement by an interrogating beam. The solid sensor andits substrate may alternatively form a waveguide structure for aninterrogation beam.

According to a second aspect of the present invention there is provideda method of manufacturing a solid sensor for detecting furfuraldehyde inoil, the method comprising the steps: of forming a colloidal suspension(sol) of methyltrimethoxysilane (MTMS) in a catalyst at an elevatedtemperature, reducing the temperature of the sol to ambient, preparingliquid aniline acetate, where the reaction temperature is maintainedbelow ambient (20° C.) during the preparation of the liquid anilineacetate, and thereafter stirring into the sol at ambient temperature aquantity of the prepared aniline acetate in liquid form so as to form asol and aniline acetate mixture, and thereafter gelating and drying themixture in air substantially at ambient temperature.

By virtue of the low temperatures used in preparing the solid sensor,and particularly the low reaction temperature during preparation of theliquid aniline acetate, the chemical functionality of the liquid anilineacetate is retained despite being encapsulated in the solid sensor.

According to a third aspect of the present invention there is provided amethod of determining the status of paper insulation in an oil-filledtransformer, comprising the steps of: providing a solid sensor formed ofa porous solidified inert matrix in which aniline acetate is entrapped,inserting the sensor into the oil for a time sufficient to allowfurfuraldehyde to react with aniline acetate encapsulated in the sensorso as to form a complex having a characteristic which is photo-opticallymeasurable and which correlates with the concentration of furfuraldehydein the oil, thereafter irradiating the sensor with an optical inputsignal of a fixed wavelength, detecting and measuring an optical outputsignal received from the sensor, comparing the output signal with areference to determine the amount of furfuraldehyde that was detected,and providing a qualitative indication of the state of the paperinsulation based on the amount of furfuraldehyde that was detected.

Preferably, an additional step of removing the sensor from the oil isperformed prior to irradiating the sensor with the optical input signalof a fixed wavelength.

It will be understood that the method also includes the step ofselecting the fixed wavelength from the range of 500 nm to 600 nm.

The characteristic which correlates with the concentration offurfuraldehyde in the oil may be the pink colour intensity of thecomplex, which is measurable by absorbance of the optical input signalso that the output signal is of the same wavelength as the input signal.Alternatively, the characteristic may be the fluorescence of the complexwhen irradiated by the optical input signal so that the output signal isof a different wavelength from the input signal.

These and other aspects of the invention will become apparent from thefollowing description when taken in combination with the accompanyingdrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a solid disc-shaped furfuraldehyde sensor in accordancewith one embodiment of the invention;

FIG. 2 shows a schematic diagram of apparatus which is used to determinethe amount of FFA in transformer oil;

FIG. 3a shows a perspective view of a glass slide coated with afurfuraldehyde sensor in accordance with another embodiment of theinvention;

FIG. 3b shows a side view of the coated glass slide of FIG. 3a;

FIG. 4 shows a solid disc-shaped furfuraldehyde sensor in accordancewith the embodiment of FIG. 1 connected to a bifurcated fibre opticbundle;

FIG. 5a shows a perspective view of a optical fibre with the usualcladding removed from a localised area of the cable and replaced with afurfuraldehyde sensor;

FIG. 5b shows a cross-sectional view of the optical fibre of FIG. 5a.

FIG. 6 illustrates fluorescence spectra derived from sensors subjectedto different concentrations of furfuraldehyde in oil; and

FIG. 7 illustrates the correlation between the furfuraldehydeconcentration and the spectra of FIG. 6.

Referring to FIG. 1, which shows a circular or disc-shaped sensor 10 fordetecting furfuraldehyde, the sensor is a solidified matrix made of aninert material in which aniline acetate is entrapped. The solidifiedmatrix is a sol-gel material which allows ingress of furfuraldehyde fromtransformer oil to react with the aniline acetate to yield an entrappedcomplex with a characteristic which is photo-optically measurable andwhich correlates with the concentration of furfuraldehyde in the oil,for example the characteristic may be the colour which is detectable bycolourimetric analysis. To produce this sensor 10, the followingprocedure is followed.

Initially, a sol is prepared. A sol is a colloidal suspension of solidparticles in a liquid. To prepare a sol to produce an FFA sensor, 28.5ml of 98% Methyltrimethoxysilane (MTMS) (available from Aldrich) isplaced in a glass jar and heated to 82° C. in an air ambient. To promotehydrolysis, a catalyst, for example 5 ml of 0.01M hydrochloric acid(HCl), is slowly mixed with the MTMS, and the solution is stirred for 5minutes. The solution consisting of about 85% by volume MTMS and 15% byvolume catalyst is placed onto a cold stirrer where it is stirred whileit cools for 2 hours. This is the MTMS sol.

If 10 ml of 0.01M HCl is used instead of 5 ml of 0.01M HCl then the solwill become cloudy in approximately 5 minutes and produce discs that aremore opaque and in some cases milky white in appearance. If 1.5 ml of0.01M HCl is used instead of 5 ml of 0.01M HCl then the discs producedare cracked, broken, deformed, and in some cases more opaque. If theMTMS sol is prepared by heating the MTMS to a temperature in the range25 to 60° C. (rather than 82° C.) then the gels that are eventuallyproduced have a milky opaque appearance which detracts from theirquality as optical devices. Raising the temperature to approximately 80°C. removes this problem of producing milky opaque gels. Raising thetemperature to above 65° C. allows evaporation of methanol (which boilsat 65° C.) which induces further benefits, such as reducing the risk ofcracking in films. The boiling point of MTMS is 102° C., therefore anacceptable range would be between approximately 75° C. and 90° C.

To prepare aniline acetate, a glass vessel containing 90 ml of 99.8%glacial acetic acid is placed in an ice bath and allowed to cool to 15°C. Once the glacial acetic acid has cooled to 15° C., 10 ml of 99% pureaniline (available from Aldrich) is slowly mixed with the glacial aceticacid; care is taken to ensure that the reaction temperature remainsbelow 20° C., although slightly higher temperatures may not have adetrimental effect on the aniline acetate produced.

To prepare an aniline acetate sol-gel, 4 ml of aniline acetate is mixedinto the MTMS sol (4 ml added to 35.5 ml is an addition of about 12% byvolume) at a temperature of 24.5° C., and stirred for 10 minutes. Toprepare discs, the wells of disposable multiwell plates, (available fromBDH) are cleaned with isopropyl alcohol. 0.5 ml of aniline acetate dopedMTMS sol is introduced into each well on a plate. The plate is coveredwith a lid and left for approximately 1 month in an air ambient atbetween 22° C. and 25° C. to gelate and dry. After the 1 month periodeach well contains a disc-shaped aniline acetate doped sol-gel which isapproximately 1 mm thick. The aniline acetate doped sol-gel can be usedas a sensor 10 because the sol-gel is porous: it allows furfuraldehydeto permeate through the sol-gel material. Thus, sol-gel material allowsingress of furfuraldehyde. Furfuraldehyde entering the sol-gel reactswith the encapsulated aniline acetate to form the pink coloured complex.

If 7.5 ml of aniline acetate is used instead of 4 ml of aniline acetatethen the discs produced are more likely to fragment, more opaque andtake longer to dry. If 10 ml of aniline acetate is used then the discsproduced are more opaque and very grainy in appearance. If 5 ml ofaniline acetate is used the discs produced are occasionally slightlyopaque.

To gelate and dry the aniline acetate doped MTMS sol in a shorter periodof time, the temperature can be raised. However, if the discs are driedin an oven at temperatures of 60° C. or more for a period of seven daysthen they are more likely to crack and are less responsive tofurfuraldehyde.

FIG. 2 shows a schematic diagram of testing apparatus 20 which is usedto determine the amount of FFA in transformer oil by detecting thecolour intensity of the coloured complex. The testing apparatus 20comprises a light source 22 which produces an input or interrogationsignal 24 which is electromagnetic radiation at a wavelength ofapproximately 530 nm. In this embodiment an LED is used as the lightsource 22. The reason that light with a wavelength of approximately 530nm is used is that the absorption of the complex of furfuraldehyde andaniline acetate has an absorption maximum at approximately 530 nm. Theinput signal 24 (electromagnetic radiation at approximately 530 nm)irradiates and is partly transmitted through a sensor 10 in the lightpath from the light source 22 to a detector 30. The detector 30 used issensitive to electromagnetic radiation at 530 nm. The output of thedetector 30 is conveyed to a processor 32 which evaluates the output ofthe detector 30 and compares the evaluated output with reference valuesstored in a reference 34.

The reference 34 contains a lookup table which is controlled by theprocessor 32. Each entry of the lookup table has a detected value oflight intensity and the corresponding amount of FFA in oil for thatvalue of light intensity. The lookup table information is loaded intothe reference 34 prior to using the apparatus 20. The processor comparesthe detected light intensity with the reference values to determine theamount of FFA in the oil. Once the amount of FFA in the oil isdetermined, the processor 32 outputs this information to an outputdisplay 36.

When transformer oil is to be tested using the testing apparatus 20,prior to inserting the sensor 10 into the oil to be tested, the sensor10 is inserted into fresh, uncontaminated oil; that is, oil which hasnever been used and so is free from any FFA. The sensor 10 is thenremoved from the fresh oil and inserted into the apparatus 20, and thelight source 22 is energised. The intensity of light received by thedetector 30 when the sensor 10 is inserted, which is the output signal38 from the sensor 10, is then recorded: this is the control value. Thecontrol value is used to scale all of the values of light intensity inthe lookup table because the lookup table values of detected lightintensity are relative to each other; they are not absolute values.

Once the control value has been determined and the look table values ofdetected light intensity are appropriately scaled, the sensor 10 is thendipped into the transformer oil under test for a preset period of time(for example 30 minutes) and then removed. The sensor 10 is dipped inthe oil for a preset period of time to ensure that the aniline acetatein the sensor 10 has had sufficient time to react with thefurfuraldehyde at the relevant oil temperature. The preset period oftime required will depend on a number of factors, such as thetemperature of the oil.

The sensor 10 is then inserted into the testing apparatus 20 and thesensor is irradiated with the input signal 24. The intensity of theoutput signal 38 received by the detector 30 is measured and conveyed tothe processor 32.

The processor 32 selects the closest value of light intensity in thelookup table to the measured value of the output signal 38 lightintensity, and reads the FFA amount entry in the lookup tablecorresponding to this entry. This corresponding FFA amount is thenoutput to the output display 36. Different types of paper may yielddifferent amounts of FFA upon degradation. Therefore, in determiningwhether the paper in a transformer has degraded by an unsatisfactoryamount, certain characteristics of the transformer (for example thetransformer size and the type of paper used) must also be considered inaddition to the amount of FFA detected in the transformer oil.

FIGS. 3a and 3 b show another embodiment of the furfuraldehyde sensor inwhich the sensor operates as a waveguide. The sensor is a coating 40 ofdoped sol-gel material which is applied to a transparent substrate, forexample a glass slide 42, where the sol-gel material is doped withaniline acetate. An input signal 24 is applied to one end of the coating(the entrance 40 a), the signal 24 passes through the length of thecoating 40, reflecting off the sides of the coating 40, and exits at theopposite end of the coating (the exit 40 b) as the output signal 38. Theoutput signal 38 is detected and measured using the detection,processing, reference, and output display apparatus described for theapparatus of FIG. 2. The output signal 38 is measured when the substrate42 and sensor coating 40 is inserted into fresh oil, and also when thesubstrate 42 and sensor coating 40 is inserted into the oil to betested. The measurements of the output signal 38 when the sensor coating40 is inserted into fresh oil and when the sensor coating 40 is insertedinto the oil to be tested are used to determine the state of the paperinsulation in the same way as described for the apparatus of FIG. 2.

FIG. 4 shows a modified sensor 10 connected to one end of a bifurcatedfibre optic bundle 50. The modified sensor has a mirrored rear face 10a, that is, there is a reflective coating applied to the face of thesensor 10 which is not in contact with the bifurcated fibre optic bundle50. The bifurcated fibre optic bundle 50 has a first arm 52, a commonarm 56 and a second arm 60. The bifurcated fibre optic bundle 50 is usedin conjunction with the light source 22, detector 30, processor 32,reference 34, and display output 36 as shown in FIG. 2. The light source22 is connected to the first arm 52 of the bifurcated fibre optic bundle50 and the detector 30 is connected to the second arm 60 of thebifurcated fibre optic bundle 50. The processor 32, reference 34 andoutput display 36 are connected to the detector 30 in the same manner asshown in FIG. 2.

Initially, a control value is generated with the modified sensor 10connected to the fibre optic bundle 50 in an ambient of fresh oil. Thecontrol value is used to scale all of the values of light intensity inthe lookup table. Once the control value is obtained and the lookuptable values have been appropriately scaled, the transformer oil istested.

To test the transformer oil, the modified sensor 10, which is stillconnected to the fibre optic bundle 50, is immersed in transformer oil.After a preset period of time (for example 30 minutes) a measurement istaken with the sensor 10 still immersed in the transformer oil. Thelight source 22 is energised and the input signal 24 propagates alongthe first arm 52 then along the common arm 56 to the modified sensor 10.The input signal 24 traverses the thickness of the modified sensor 10before being reflected from the rear face 10 a of the modified sensor 10and again traversing the thickness of the modified sensor 10 beforeemerging as the output signal 38. The output signal 38 is propagatedalong the common arm 56 then the second arm 60 of the bifurcated fibreoptic bundle 50 into the detector 30. The output signal is used toaccess the lookup table in the same way as described for the apparatusof FIG. 2.

The relevant FFA amount entry from the lookup table is then output tothe output display 36.

This arrangement has the advantage that the modified sensor 10 is fittedto the bifurcated fibre optic bundle 50 prior to immersion into the oil,therefore the test is conducted without having to remove the modifiedsensor 10 from the oil.

FIGS. 5a and 5 b show another embodiment of the furfuraldehyde sensor inwhich the sensor is part of the cladding of an optical fibre. Theoptical fibre 70, such as a plastic-clad silica fibre, has its outercladding 72 completely removed from a portion of its length and replacedwith a furfuraldehyde sensor coating 40.

The FFA sensor coating 40 is applied by dipping the fibre 70 intoaniline acetate doped MTMS sol. The fibre 70 coated with aniline acetatedoped MTMS sold is then left to gelate and dry for a suitable time. Oncethe sol has gelated and dried the thickness of the coating 40 applied tothe fibre optic is measured. If the coating 40 is not thick enough thenthe procedure is repeated, namely the coated fibre is dipped intoaniline acetate doped MTMS sol and allowed to gelate and dry. When thecoating 40 has reached the desired thickness, the fibre is ready for usein detecting FFA. The desired thickness may be the thickness thatproduces the highest absorption (for a given FFA concentration in theoil) of the input signal 24 as it travels along the optical fibre 70.

An input signal 24 is applied to one end (the fibre entrance 70 a) ofthe fibre 70. The intensity of the signal that exits at the opposite endof the fibre (the fibre exit 70 b) as the output signal 38 is less thanthe intensity of the input signal 24 because a portion of the signal wasabsorbed by the original cladding 72 and by the sensor coating 40. Priorto immersing the coated fibre in transformer oil containing FFA, thesensor coating 40 and the original cladding 72 will not absorb much ofthe input signal 24. After immersing the coated fibre in transformer oilcontaining FFA, however, the sensor coating 40 will absorb significantlymore of the input signal than the original cladding 72.

The output signal 38 is detected and measured using the detection,processing, reference, and output display apparatus described for theapparatus of FIG. 2. The output signal 38 is measured when the sensorcoating 40 on the fibre 70 is inserted into fresh oil, and also when thesensor coating 40 on the fibre 70 is inserted into the oil to be tested.The measurements of the output signal 38 when the sensor coating 40 isinserted into fresh oil and when the sensor coating 40 is inserted intothe oil to be tested are used to determine the state of the paperinsulation in the same way as described for the apparatus of FIG. 2.

One advantage of having the aniline chemistry immobilised in a sol-gelmaterial is that the colour of the complex produced by the reaction ofthe aniline acetate with the furfuraldehyde is retained for at least oneweek; whereas the chemical solutions currently used in industry retaintheir colour for only a few minutes. The advantages of colour retentionare that the test can be repeated a number of times, and the delaybetween removing the sensor 10 from the oil and performing the test isnot critical.

A further advantage of using a sol-gel material is that there is nodeterioration of the sol-gel material even if it is left in oil for aperiod of two months. Thus, the sol-gel material will not introducechemical substances into the oil if it is left in the oil for slightlylonger than is needed to perform the test.

The characteristic of the coloured complex to be measured may be thefluorescence of the complex when irradiated by an optical input signal.This is achieved by irradiating the side of the disc sensor with theinput signal preferably having a wavelength of about 525 nm andmeasuring the resultant wavelength-shifted output signal emitted fromthe face of the sensor. The output signal, in this case, has a maximumamplitude at about 570 nm and the numeric value of that amplitudecorrelates with the concentration of FFA in the oil. For example, FIG. 6shows the resultant fluorescence spectra for sensor discs immersed inoil containing different concentrations of FFA of 1.0 ppm and less. Ifthe zero ppm signal amplitude at 570 nm is taken as a base, the relativemagnitudes of the remaining signals show a linear relationship with FFAconcentration as shown in FIG. 7.

Although FIGS. 6 and 7 illustrate the fluorescent effect between the 525nm and 570 nm wavelengths, any other set of fluorescent wavelengths maybe utilised to provide the required photo-optic measurements. As is wellknown, fluorescence measurements have a fundamental signal-to-noiseadvantage over absorption measurements because background radiation atthe wavelength of measurement is minimal.

Various modifications may be made to the above described embodiments.For example, the light source may emit light with a fixed wavelengthother than 530 nm, but preferably in the range from 500 nm to 600 nm.Similarly, other embodiments may use a light source other than an LED,for example a filament lamp may be used in conjunction with a bandpassfilter to provide a light source in the range 500 nm to 600 nm.

In the FIG. 2 embodiment described a single sol-gel disc is used, inother embodiments more than one sol-gel disc (for example eight sol-geldiscs, all in the path of the input signal) could be used to increasethe colour contrast and hence increase the absorption of the inputsignal. This has the effect of reducing the intensity of the outputsignal, thus increasing the difference between the input signal and theoutput signal.

In the embodiments described hydrochloric acid is used to promotehydrolysis in producing an MTMS sol, in other embodiments other suitableacids, alkalis (for example sodium hydroxide), or catalysts may be usedinstead.

In the embodiments described a sol-gel is used, in other embodimentsother suitable chemical processes may be used to encapsulate anilineacetate in an inert material, providing the material allows ingress offurfuraldehyde. In the embodiments described a circular sol-gel disc isused, in other embodiments a square or rectangular shape of sol-gelmaterial may be used.

In the embodiments described, the 30 minute period for which the sol-gelmaterial is dipped in oil is one suitable time period; however, anyconvenient period of time may be used provided the sol-gel material hasadequate time to form the coloured complex. In other embodiments thetransformer oil may be heated (e.g. 90° C. oil) to reduce the time takenfor the sol-gel to form the coloured complex. In the embodimentdescribed, an MTMS-based process is used to produce a sol-gel material,in other embodiments a process based on tetraethylorthosilicate (TEOS)may be used. Other suitable quantities of chemicals may be used toproduce a sol-gel material.

In the embodiments described a lookup table is used in the reference, inother embodiments the processor may execute an algorithm which uses apolynomial approximation to map the detected light intensity to thecorresponding amount of FFA in the transformer oil. In other embodimentsof the invention, the control value for the sensor is obtained in anambient of air rather than the ambient of fresh oil as described in theabove embodiments.

In other embodiments of the invention, a coating is applied to theoutside of an optical fibre as an area which has a reduced thicknesscladding. An input signal is applied to one end of the fibre and anoutput signal normal to the fibre is detected and measured.

In the embodiment shown in FIGS. 5a and 5 b, the entire length ofcladding may be removed from the optical fibre and replaced with asensor coating 40. The optical fibre 70 may be coiled to allow a largeamount of fibre to be inserted into a small volume of transformer oil.

In the embodiment shown in FIGS. 5a and 5 b, a single fibre 70 is usedto obtain a control value of light intensity when the fibre 70 is dippedin fresh oil and then a test value of light intensity when the fibre 70is dipped in the transformer oil to be tested; an alternative embodimentuses two fibres with identical sensor coatings 40. One of the fibres isinserted into fresh oil and the other fibre is inserted into thetransformer oil to be tested. Identical input signals 24 are applied toeach fibre and the respective output signals are compared to determinethe state of the paper insulation.

Similarly, in the embodiment shown in FIGS. 3a and 3 b, a singlesubstrate coated with an FFA sensor is used, an alternative embodimentuses two identical coated substrates. One of the coated substrates isinserted into fresh oil and the other coated substrate is inserted intothe transformer oil to be tested. Identical input signals 24 are appliedto each coated substrate and the respective output signals are comparedto determine the state of the paper insulation.

What is claimed is:
 1. A solid sensor for detecting furfuraldehyde inoil, where the sensor comprises a solidified matrix in which anilineacetate is entrapped, and where the matrix is made of an inert materialwhich allows ingress of furfuraldehyde from the oil to react with theaniline acetate to yield an entrapped complex having a characteristicwhich is photo-optically measurable and which correlates with theconcentration of furfuraldehyde in the oil.
 2. A solid sensor as claimedin claim 1, wherein the matrix material is a sol-gel.
 3. A solid sensoras claimed in either preceding claim and which has sufficient thicknessto be self-supporting.
 4. A solid sensor as claimed in claim 1 or claim2 and which comprises a coating carried by a substrate.
 5. A method ofmanufacturing a solid sensor for detecting furfuraldehyde in oil, themethod comprising the steps: of forming a colloidal suspension (sol) ofmethyltrimethoxysilane (MTMS) in a catalyst at an elevated temperature,reducing the temperature of the sol to ambient, preparing liquid anilineacetate, where the reaction temperature is maintained below ambient (20°C.) during the preparation of the liquid aniline acetate, and thereafterstirring into the sol at ambient temperature a quantity of the preparedaniline acetate in liquid form so as to form a sol and aniline acetatemixture, and thereafter gelating and drying the mixture in airsubstantially at ambient temperature.
 6. A method as claimed in claim 5,where the sol is produced at a temperature in the range 75° C. to 90° C.from about 85% by volume MTMS and 15% by volume catalyst in the form of0.01M hydrochloric acid, and the mixture is formed by adding about 12%by volume of aniline acetate.
 7. A method of determining the status ofpaper insulation in an oil-filled transformer, comprising the steps of:providing a solid sensor formed of a porous solidified inert matrix inwhich aniline acetate is entrapped, inserting the sensor into the oilfor a time sufficient to allow furfuraldehyde to react with anilineacetate encapsulated in the sensor so as to form a complex having acharacteristic which is photo-optically measurable and which correlateswith the concentration of furfuraldehyde in the oil, thereafterirradiating the sensor with an optical input signal of a fixedwavelength, detecting and measuring an optical output signal receivedfrom the sensor, comparing the output signal with a reference todetermine the amount of furfuraldehyde that was detected, and providinga qualitative indication of the state of the paper insulation based onthe amount of furfuraldehyde that was detected.
 8. A method as claimedin claim 7, wherein the optical output signal has the save wavelength asthe optical input signal and the measurable difference is due toabsorption.
 9. A method as claimed in claim 7, wherein the opticaloutput signal has a different wavelength to the optical input signal andthe measurable characteristic is due to fluorescence.
 10. A method asclaimed in claim 8 or 9, wherein the optical input signal has awavelength in the range 500 nm to 600 nm.